In the 1920's to 1940's, electrical took a turn to a more protective wiring sceme, flexible armormed cable. Flex, also known as Greenfield, was a welcomed addition to home wiring because the flexible metal walls helped to protect the wires from damage. Even then, this wiring method had its troubles. Although the wire is protected and the outer flexible metal cover acts as a ground, there still was no seperate ground wire. If the flexible covering didn't make contact with the next piece or it was cut, the ground connection was severed.
In the 1930's, a quicker installation method was developed. Nonmetallic sheathed cable was born and it incorporated a rubberized fabric coating sheath, much like knob and tube wiring, but a hot and neutral wire were run together in this one sheath. It also had its drawbacks due to the lack of a ground wire.
Luckily in the 1940's, finally came the age of metal conduit. This invention allowed users to pull many wires in the same enclosure. The conduit itself is considered a grounding method, but also leaves the possibilty of space for a ground wire to be pulled. Conduit has been in use ever since those days and comes in many different types and sizes to be used inside and outside of your home.
The newest addition to wiring was introduced in the 1960's around 1965. It was an update to NM cable that incorporated the use of a third wire, a bare ground wire run with a hot and netral wire. These three wires are all concealed in an outer sheath made of plastic vinyl. This update made the cable inexpensive and very easy to install. It is very flexible and is used still today.
Along with NM cable for interior use, a similar type cable was also invented. Underground feeder wire(UF) was invented to be buried directly under the ground without having to be placed in conduit. This type wire has a hot, a neutral, and a ground wire embedded in a solid plastic vinyl sheath that protects it from damp areas, water, and materials underground. This was an inexpensive addition to running power underground to things like yard lights and outbuilding feeds.
As you can see, things have certainly changed over the years for the better! You see, the rubber-coated wire would only hold up for 25 years or so before the rubber would dry out and start cracking. This left exposed bare wires that could cause a multitude of problems. Plastic vinyl has been shown to last the life expectancy of your home and is a much better method of wiring. For the preferred method in my book, I choose a conduit installation. With this, you can pull the wires easily, add wires to the conduit (as allowed), and have a system that protects the wires from damage. It is the all-in-one system, of choice. The only question now is, will we see another next big thing in electrical wiring in our lifetime?